Methoding of mounting a partly formed lens blank

ABSTRACT

The invention relates to a machine for aligning and attaching partly-worked lens blanks to a block so that the remainder of the blank can be machined. The machine comprises a vee-shaped holder for supporting the block with it axis vertical above a moveable table on which the lens blank is supported. The moveable table is freely moveable in any direction in a horizontal plane and the machine includes means for bringing the block and partly-worked lens into contact and achieving alignment of the axes of the block and lens blank by permitting the lens blank to move in any direction in a horizontal plane. A film of liquid adhesive is provided between the surface of the lens blank and the block which acts both as a lubricant and as a bonding agent when correct alignment has been achieved.

This invention relates to the manufacture of lenses, especially contactlenses.

Contact lenses are conventionally manufactured by machining suitablysized lens blanks on automatic or semi-automatic lathes. Althoughaccording to some machining procedures the lens blank itself is receivedinto the collet of the lathe the more commonly adopted methods involveattaching the lens blank to a chuck which is then received in the lathecollet. After the first curved lens surface has been cut on the blank itis necessary to remove the partly machined blank from the chuck and tothen attach it to the second chuck (known in the contact lens art as a"block") so that the opposite lens surface can be formed. This procedureof mounting a partly machined lens blank on a block is known in the artas "blocking". One difficulty in carrying out the blocking operation isto ensure that the optical axis of the partly machined blank is alignedwith the axis of the block so that the second lens surface will be cuton the same axis as the first. This procedure is conventionally carriedout by manually aligning the blocks and the lens blank and isaccordingly an extremely laborious and skilled operation. Failure toachieve accurate alignment of the lens blank and block will lead to therejection of the lens in final checking owing the poor edge control orprism.

According to the present invention there is provided a method ofmounting a partly formed lens blank, having a first lens surface, on ablock in order to be able to cut a second lens surface on the blank,said method comprising bringing the block into contact with the firstlens surface of the partly formed, e.g. machined, blank whilemaintaining the axes of the block and blank in a vertical dispositionand, while supporting the lower component of the block or blank so thatit is free to move in any direction in a plane at right angles to theaxes of the block and blank, urging the lens surface and block firmlytogether with a uniform film of liquid adhesive therebetween, wherebysaid liquid film acts as a lubricant between the mating surfaces of theblock and the lens surface and assists centralisation and alignment ofthe axes of the blank and block.

The block is conveniently lowered onto a blank, which is supported sothat its optical axis is maintained in a vertical disposition, whilebeing free to move to a limited extent in a horizontal plane. If thepartly machined lens blank has been machined to form the first lenssurface while supported on a chuck, the blank may be transferreddirectly from its associated chuck to a block.

According to a further aspect of the invention there is provided ablocking machine for centring a partly machined lens blank having afirst lens surface to a block in order to machine the opposite lenssurface, said machine comprising a holder for supporting one of thecomponents comprising the block and lens blank above a moveable tablefor supporting the other component, said table in at least one mode,being free to move in any direction in a horizontal plane and means formoving the holder and table towards each other and bringing the firstlens surface and block into mutual contact. Preferably, the lens blankis attached to a chuck and the lens blank and chuck supported by thetable, while the block is mounted in the holder and lowered under theweight of the holder and block onto the lens blank. The table on whichthe lens blank and chuck is supported is arranged to be freely moveablein a horizontal plane and this can be achieved by supporting the tableon ball-bearings. Preferably the table includes means for selectivelylocking and unlocking the table so that after the transfer operation,the table can be held solidly.

While the chucks and blocks can be loaded by hand into the holder andtable the blocking machine is preferably designed to be loaded andunloaded automatically. The apparatus may include means forautomatically applying adhesive to the block and means for breaking thebond between the chuck and the lens after the blank has been bonded tothe block.

Further features and aspects of the present invention will becomeapparent from the following description and accompanying drawings.Various embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 is a side elevation of the blocking apparatus showing the blockin a position in which the block has almost been lowered to contact thelens blank,

FIG. 2 is a view taken in the direction of the arrow A in FIG. 1,

FIG. 3 is a view taken on the line B--B in FIG. 1 but showing inaddition clamping means for selectively locking the table,

FIG. 4 is a side elevation of a chuck with a partly machined lens blankadhered to it,

FIG. 5 is a side elevation of a block after the partly machined blankhas been transferred to it,

FIG. 6 is a schematic view of the blocking apparatus incorporated in anautomatic machine.

Referring to FIGS. 1 to 5 of the drawings, the blocking apparatuscomprises a carriage 1 for supporting a block 2 vertically above a table3. Carriage 1 is guided on a slide 4 and may be raised or lowered on theslide 4 manually or by automatic means, such as by a wheel 5 arranged torun on a cam surface (not shown). Carriage 1 comprises a V-shaped block6 and a pair rollers 8 carried on arms 9 for supporting the block 2.

Table 3 comprises an upper part 10 which is supported on a base 11 bymeans of a bed of ball-bearings 12 (only some of which are shown inFIGS. 1 and 3) which allow a limited degree of horizontal slidingmovement between the upper part 10 and the base 11. As shown best inFIG. 3 the upper part 10 of table 3 includes a fork portion 13 forreceiving the chuck 14 and its associated lens blank 15 (for clarity thechuck and lens blank are not shown in FIG. 3).

As can be seen in FIG. 1, the upper part 10 of the table has a dependingskirt 21 which surrounds the major portion of the base part 11. Incommon with the upper part 10, the base part 11 is generally U-shapedwhen viewed in plan (see FIG. 3) so as to accommodate the body of chuck14. Ball bearings 12 are located on base part 11 in groups on each sideof recess 22 for receiving the body of chuck 14. Upwardly projectingwalls 23 are provided to retain the balls in beds on each side of recess22. The upstanding walls 23 trap the balls 12 in the space between themutually slideable parts of the table 3 but without restricting theirrolling movement. Depending skirt 21 of the upper part 10 limits theextent of mutual horizontal movement of the components of the table,although the upper part 10 is free to move in any direction in ahorizontal plane.

The forked portion 13 is open towards the left hand side of theapparatus as shown in FIG. 1 and supports the chuck on the under face ofthe lens blank 15, the adherent chuck hanging from the forked portion13. During the first machining operation for cutting the base curve 16,the surface 17 is accurately cut at right angles to the axis 17b of thebase curve 16.

The procedure for transferring the lens blank 15 from the lower chuck tothe upper chuck or block 2 is as follows. With the lower chuck supportedon the fork portion 13 of the table 3 and with the table 3 in itsfloating mode so that the upper part 10 of the table is free to move inany direction in a horizontal plane on the bearings 12) the carriage 1is moved down on its slide until the rounded surface of the block 2touches the base curve 16 of the partly cut blank. Prior to thismovement the rounded surface of the block 2 will have been heated andreceived a measured amount of molten thermoplastic adhesive such as awax-based adhesive which has been heated to melt it. At this stage therounded end of the block 2 will be resting on the base curve 16 underthe weight of the carriage but guided in by the Vee-block 6 in avertical disposition. The apparatus is arranged so that the axis of theVee-block 6 is accurately at right angles to the plane of the upper part10 of the table 3 and the block 2 is accurately machined as a cylinder.As a consequence of the freedom of the table 3 to move in any directionin a plane parallel to the accurately machined surface 17 of the blank15, the lens blank 15 adjusts and centralises its position relative tothe block 2 so that the axis of the latter lies accurately along theaxis on wich the base curve 16 had been cut. The molten adhesive betweenthe block 2 and the surface 16 acts as a lubricant during thiscentralising movement. Once the adhesive on the end of the block 2 hascooled the composite structure of the block, chuck and lens blank can beremoved as one from the apparatus. The bond between the chuck 14 and theblank 15 can then be broken so that the block 2 carrying the blank 15(see FIG. 4), can then be inserted into the collet of a lathe and apower curve cut on the front surface of the blank.

It will be appreciated that the block and lens blank should be broughttogether in such a way that a thin, uniform film of molten adhesiveexists between the contacting surfaces while the centralising actiontakes place and before the adhesive cools to form a bond. Obviously theblock should not be at a temperature which is so high as to damage thesurface of the lens surface. Using a blended resin/wax adhesive it hasbeen found that a suitable temperature for the block is about 90° C.,just above the melting point of the adhesive, although the optimumtemperature depends on the particular adhesive selected and the natureof the lens polymer. It is currently preferred to bring the block fairlyrapidly into contact with the lens blank, by allowing the carriage torun down on a steep cam surface under its own weight. These factors areall related and some experiment will be necessary to arrive at theoptimum conditions for a particular choice of lens polymer, adhesive andheat capacity of the lens blank.

As mentioned above the blocking machine may be part of an automaticapparatus which is fed sequentially with blocks and with chucks havingadherent lens blanks and the resulting blocks and transferred blanks areunloaded subsequently. Such automatic apparatus may be incorporated in alens manufacturing procedure as described in our British patentapplication No. 8313161 (publication No. 2123733) and it may utiliseloading and unloading devices similar to those described in our aboveapplication.

It is desirable, although not essential, during subsequent operations,e.g. when splitting the chuck from the lens blank, to lock the table 3.Pivotally mounted on a shaft 24 is a clamping arm 25, the shaft 24 beingjournalled in trunnion blocks 27 secured to base plate 26. Attached tothe skirt 21 or upper part 10 of table 3, is a horizontally outwardlyprojecting web 28. Pivotal movement of clamping arm 25 towards the baseplate 26 presses down on the web 28 and holds the upper part 10 of table3 rigidly. A spring-biassed latch 29 locks the clamping arm in theclamped position. The upper part 10 of table 3 can be instantly releasedby disengaging the latch 29, which is formed with a detent to hold thearm 25 up in its disengaged position.

FIG. 6 is a schematic plan view of such an automatic blocking apparatuswhich consists of a pair of carousels 101 and 102 rotatable in thedirection of the arrow. Each carousel has a number of stations at whichthe operations to be described are carried out and means are provided torotate the carousels in timed sequence from one station to the next.Blocks 2 (as shown in FIG. 1) are loaded (with their rounded endsprojecting downwardly below the plane of the carousel) one after anotherat station 103 to carousel 102 using a star-wheel loading device 104 ofthe kind illustrated in FIG. 6 of our above-mentioned British PatentApplication. Carousel 102 incorporates a heater so that by the time theblocks have reached station 105 their temperature is higher than themelting point of a waxy thermoplastic adhesive. This is applied atstation 105 by lifting a bath of molten adhesive into contact with therounded end of the block. At station 106 the blocks are transferred intothe open arms 9 of the Vee-block 6 which is mounted on the carousel 101.Devices 110 are wedges which are arranged to lock the carousels aftereach indexing step. Carriage 1 moves down on slide 4 under the controlof the wheel 5 sliding on a cam surface and the table 3 at this point isunlocked so that the upper part 10 is floating on the bearings 12. Therounded tip of the block is lowered onto the surface 16 of the lensblank. In this condition of the apparatus, the block hangs vertically inthe Vee-block 6 and with the table 3 free to move in a horizontal plane,the block is centralised and aligned with respect to the lens blank.After the adhesive has cooled the lens and its adherent block and chuckmove to the next station (107) at which the bond between the lens blankand the chuck is broken by means of a thin wedge-shaped tongue 111 whichprojects in one sharp movement between the fork portion 13 and theremainder of the upper part 10 of the table 3. Chucks are ejected aftersplitting the bond with the lens blank. Upper part 10 of table 3 islocked just before the tongue breaks the bond between the chuck andblank. Blocks and adherent lens blanks are removed by a star wheelunloading device at station 108 into magazines for transfer to the frontcurve lathe. Station 109 also includes a star wheel loading device toload chucks and adherent blanks from a magazine to table 101.

We claim:
 1. A method of mounting a partly formed lens blank, having afirst accurately cut lens surface, on an upper block in order to be ableto cut a second opposite lens surface on the blank, said methodcomprising bringing the upper block into contact with the firstaccurately cut lens surface of the blank, said blank being secured on alower support component, while maintaining the axes of the upper blockand blank secured on said lower support component in a verticaldisposition and, while supporting the lower support component and blankso that it is free to move in any direction in a horizontal plane,urging the first accurately cut lens surface and upper block togetherwith a film of liquid adhesive therebetween, whereby said adhesive actsas a lubricant between mating surfaces of the upper block and the firstaccurately cut lens surface and assists centralisation and alignment ofthe axes of the blank and upper block, and removing said lower supportcomponent from said lens blank.
 2. A method according to claim 1 whereinthe upper block and first accurately cut lens surface are urged togetherby virtue of the weight of the upper component of the lens block.
 3. Amethod according to claim 1 or claim 2 wherein said lower supportcomponent is a chuck and said first lens surface is a machined surface.4. A method according to claim 3 wherein the upper block is lowered ontothe lens blank with the latter supported in an upright attitude on saidlower support component while being free to move in a horizontal plane.5. A method according to claim 4 wherein the lens blank is supported inor on a table while mounted for limited horizontal movement on bearings.6. A method according to claim 1 wherein said adhesive is a hot meltadhesive and is applied to the block or blank just prior to bringingthem together.
 7. A method according to claim 4 wherein the sequence ofoperations comprises feeding the upper block to a vee-shaped support,lowering the support onto the lens blank with a liquid adhesive betweenthe mating surfaces, causing centring of the lens blank with respect tothe upper block by permitting the lens blank to move relatively to theupper block in any direction in a horizontal plane under the loadapplied by the upper block and its support, and allowing the adhesive toset.
 8. A method according to claim 7, wherein the lens blank issupported in a forked member mounted for sliding movement in ahorizontal plane while the block and lens blank are urged together.